30G 



THE PENOKEE IKON-B EARING SERIES. 



crystals. One of" the slates has a peculiar mottled appearance when 

 viewed under the microscope, resembling that presented by raindrops on 

 sliale, but the spots are indefinitely smaller. They seem to be due to tlie 

 relative proportions of the minerals which comj)ose them. Tliis aiTange- 

 ment may be caused by the decomposition of feldspar, each one of the 

 lighter spots perhaps representing a rounded fragment of that mineral. 



In order to reenforce the microscopical determination of tlie minerals in 

 these obscure slates, analyses of two of the magnetitic ones were made in 

 the U. S. Geological Survey laboratory l)y Mr. L. G. Eakins, with the result 

 of confirming fully the observations made. The first specimen is from NW. 

 i Sec. 6, T. 45 N., R. 2 E., Wisconsin; the second from NE. \ Sec. 1, T. 45 

 N., R. 1 E., Wisconsin. The amount of magnetite would appear in lioth 

 cases to be as much as 15 per cent. 



Analijavs of slates. 



SiOj 



AlA 



Fe^Os 



FeO 



MnO 



CaO 



MgO 



KjO 



N:t;0 



LijO 



H/) 



TjOs 



Total 



II. 



53-44 



19-62 



ll-;i8 



5-35 



trace 



-42 



1-58 



1-73 



2-61 



trace. 



1-07 



trace. 



100-20 



52-58 



20-76 



12-17 



4-08 



-21 



■30 



1-33 



4-87 



-37 



trace. 



3-43 



The rjraifWKckes and (jmi/wacke-slates cover much the largest territory 

 of any class in the Uj)per slate mend)er. The graywackes' have always as 

 cliief constituents fragmeutal quartz and felds})ar. The strengtli of the 



' Till) tiTiii ^ray wiickf is here usotl iu :i lithological seuw, iu ,-ii-cordaiicc witli the (U^linitioii i>f ( he 

 trrm Kivi-n by GciUic, Text liook ipf Grology, 2(1 ud., ji. 162: " A coinpact aj^grcKati- of rouudcd or .siili- 

 auf^tilar f^raiii.s oC (luartz, fi'lds])ar, slate, or other minerals; or roeks, comeiiteil by a i)ast.e, wliieli is 

 iKsually siliceous, but may bi! arf^illaei-ous, I'eldspathie, calcareous, or authraeitic. Gray, as itsiuime 

 denotes, is its ll^(^vailillK■ color; but it jiasses into brown, brownish i>nr]de, anil soml^tiMles, where 

 anthracite nrodomiuates, into black. The rock is distini^nishod from ordinary sandstone by its darker 

 line, its hardness, the variety of its comixmeut grains, and above all by the comjiact cement in which 

 the grains are embedded." 



